One known example of this type of lighting device is a so-called “LED in glass” device. An example is shown in FIG. 1. Typically a glass plate is used, with a transparent conductive coating (for example ITO) forming electrodes. The conductive coating is patterned in order to make the electrodes, that are connected to a semiconductor LED device. The assembly is completed by laminating the glass, with the LEDs inside a thermoplastic layer (for example polyvinyl butyral, PVB).
Applications of this type of device are shelves, showcases, facades, office partitions, wall cladding, and decorative lighting. The lighting device can be used for illumination of other objects, for display of an image, or simply for decorative purposes.
One problem with this type of device is that the layer which encases the LEDs within the structure should not be overheated, because otherwise it will lead to damage in the layer (for example PVB). Additionally also the LED itself should not be overheated (in particular the junction temperature should not become too high), because otherwise the LED lifetime goes down. The glass itself is typically not sufficiently thermally conducting to sink the heat away, so additional precautions have to be taken. Currently the maximum allowed power for the LEDs is limited (for example to 0.3 Watt). By limiting the operating current of the LEDs, a visible shift in color temperature can result. Hence, for cost as well as performance, the LEDs should be operating close to their nominal power which is currently not possible when using for example 1 Watt LEDs.
It is an object of the invention to provide a structure which allows the LEDs to be operated at a desired power level without risking damage to the device structure.